A patch antenna has a typical structure of a flat antenna. The patch antenna uses a rectangular or circular metallic pattern formed on a surface of a dielectric substrate as a radiator, the metallic pattern resonating in radio frequency signals sent or received. The patch antenna uses a metallic film formed on a back surface of the substrate as a ground electrode. Since general patch antennas have a ground electrode on the back surface, they exhibit the directivity that radio waves are directed to a surface (front) direction of the antenna. Because of this characteristic, the patch antennas are often used in applications in which they are stuck to the surface of equipment or a wall to transmit and receive radio waves in the direction toward the front of the antenna. However, when the size of the ground electrode of the patch antennas is small, the directivity of the antennas is insufficient for radiation in the front direction, so that some radio waves leak to sides and the rear, possibly resulting in interference.
For suppressing unnecessary radiation to sides and the rear in a patch antenna, A high impedance plane (HIP), a photonic band gap (PBG), or an electromagnetic band gap (EBG). Since HIP, PBG and EBG basically have similar structures.
As described in U.S. Pat. No. 6,262,495, in the EBG polygonal (e.g., hexagonal) metallic electrodes are cyclically disposed on the surface of a dielectric substrate so that the metallic electrodes are electrically connected with a metallic film formed on the back surface of the dielectric substrate through connection materials within via holes penetrating through the dielectric substrate. In the EBG, since the above structure exhibits the characteristics of a circuit in which inductors (L) and capacitors (C) are continuously connected, an LC resonance occurs in a specific frequency and impedance becomes high when a radio frequency signal transfers through the surface. The frequency area in which impedance becomes high is-called a band gap.
When this phenomenon is combined with a patch antenna 30 as shown in FIGS. 18A and 18B so that EBGs are disposed in the vicinity of the patch antenna 30 to bring the resonance frequency of the patch antenna 30 into agreement with that of EBGs 31, a radio frequency signal radiated from sides of the patch antenna 30 can be attenuated by the resonance effect of the EBGs 31. As a result, the invasion of radio waves into sides and the rear of the patch antenna 30 is suppressed and unnecessary radiation can be suppressed. In FIG. 18B, the reference numeral 32 designates a coaxial cable. Detailed characteristic results of the above construction are reported in Matsugatani, et al., “Radiation Characteristics of Antenna with External High-Impedance-Plane Shield,” the Institute Electronic, Information and Communication and Engineers English Papers IEICE Trans. Electron, Vol E86-C, No. 8, Aug. 2003, p. 1542-1549.
Thus, by combining the EBG and the patch antenna, an antenna can be provided with a thin shape and excellent directivity. However, in the case of the above construction, a frequency bandwidth usable as the antenna becomes narrow. This is attributed to the principle of the patch antenna itself. The patch antenna uses a resonance phenomenon of metallic electrodes formed on a dielectric substrate, and very sharp resonance occurs due to a confining phenomenon of an electric field oriented from ends of the metallic electrodes to the dielectric. As a result, despite the excellent radiation characteristics, the width of resonance frequencies, that is, a frequency width usable for transmission and reception as an antenna becomes very narrow.
Moreover, in the case of combining a patch antenna and EBG, the patch antenna is based on a resonance phenomenon due to a geometrical shape of metallic electrodes, but EBG is based on an LC resonance phenomenon. Therefore, a complicated design is required to bring their resonance frequencies into agreement with each other.